1. Field of the Invention
The invention relates to transmission devices for a cycle. In particular, the invention relates to a transmission device with gear shift.
2. Description of Background
For a cycle, such as a bicycle, the transmission includes the assembly of mechanical means by which the cyclist's pedaling force is transmitted to a driving wheel.
Such transmissions can include those means required for changing gears or, to be more precise, the means for changing the gear ratio. Such means were implemented as of the invention of the pedal-equipped velocipede in the second half of the 19th century. The overall transmission ratio, which is hereafter referred to as the gear ratio of the transmission device, corresponds to the rotational speed ratio between “the input”, i.e., the pedaling speed, and the “output”, i.e., the rotational speed of the driving wheel. In cycles whose pedaling axis is not merged with the driving wheel axis, the modification of the transmission ratio can be carried out either in the area of the bottom bracket, or in the area of the driving wheel, or at both locations.
In a conventional bicycle, the rear wheel is the driving wheel and the pedaling axis is positioned forward of the driving wheel. For reasons of clarity in the following description, the expression “gear ratio” refers to the ratio between the angular speeds of pedaling and the rear wheel, whereas “transmission ratio” refers only to what occurs in the area of the crankset.
Over the years, cycle manufacturers have proposed a number of transmission devices including ratio changing means. The most widely used ratio changing means include one or more toothed chainwheels driven by the pedals at the input, and one or more toothed sprockets which drive the rear wheel at the output. A hollow-link chain connects the input to the output. The ratio change, strictly speaking, involves changing the position of the chain relative to the sprockets and/or to the chainwheels. This shift of the chain position is carried out by one or more derailleurs.
The patent documents EP 1 944 230 and US 2008/0167148 disclose a front derailleur that enables the position of the chain to be changed relative to the chainwheels. In such a device, the change in the position of the chain occurs on the tension side the chain. This is why it is not possible, or not recommended in any case, to operate the front derailleur when a force is being exerted on the pedals. In practice, the user releases the force before and during each ratio change. The loss of efficiency for the cyclist due to the derailleurs is therefore understood.
In a conventional bicycle according to the prior art and, in particular, in a bicycle equipped with a front derailleur, such as that disclosed in the patent documents EP 1 944 230 and US 2008/0167148, the transmission device thereof includes a crankset equipped with two or three front chainwheels, a chain, and a rear sprocket cassette. The means for changing the gear ratio include a front derailleur associated with a front control lever positioned on the left portion of the handlebar, and a rear derailleur associated with a rear control lever positioned on the right portion of the handlebar.
The two front chainwheels include 39 and 53 teeth, respectively, whereas the cassette includes 10 rear sprockets including 11, 12, 13, 14, 15, 16, 17, 19, 21, and 23 teeth, respectively. This is a conventional configuration for a road bicycle. Each gear ratio is constituted by the association of a front chainwheel and a rear sprocket. Theoretically, 20 transmission ratios can be envisioned with such a device (2 chainwheels multiplied by 10 sprockets). In practice, there are only approximately 16 distinct transmission ratios, as can be seen in Table 1 below.
TABLE 1
In Table 1, each line corresponds to one of the twenty gear ratios. Each column contains from left to right, respectively, the number of the ratio, the number of chainwheel teeth, the number of sprocket teeth, the numerical value of the gear ratio, and the variation, expressed as a percentage, separating this numerical value from the value of the preceding line.
The gear ratio is equal to the ratio of the number of chainwheel teeth to the number of sprocket teeth.
Several weaknesses of this transmission system can be emphasized. First, as shown in Table 1, the same transmission ratios are repeated several times. Moreover, some of these transmission ratios cannot be used in reality because of the chain misalignment between the chainwheels and the sprockets of the rear wheel. This generates friction of the chain on the front derailleur, the noise from which is extremely unpleasant for the cyclist, and causes a drop in efficiency and increased wear on the chain. In addition, the transitions between the various ratios are not always very progressive. This can be estimated using a variation percentage between two consecutive ratios. Beyond 10% (as shown in bold face in the table), the transition is not considered to be progressive at all. Finally, it is not always easy to operate a change of chainwheel while the cyclist is exerting force because the front derailleur acts on the tensioned side of the chain, which causes a break in the pedaling rate, and can have a negative effect for the cyclist, particularly in professional cycling.
In conclusion, it is not easy for the user to have an optimal control of such a system for changing ratios, because cyclist must be aware of the positioning of the chain—which chainwheel and which sprocket are involved—before each change of ratio. Indeed, the control action to undertake, whether for actuating the control of the front, of the rear, or of both derailleurs, is dependent upon this positioning.
The patent documents EP 1 145 947 and U.S. Pat. No. 6,537,173 disclose a device that overcomes some of the weaknesses of conventional transmission devices. This device uses a single toothed chainwheel which, because it is expansible, fulfills the same function as two chainwheels. However, the use of expansible chainwheels requires the installation of complex mechanisms with numerous moving parts which must withstand and transmit the pedaling torque. In practice, these systems were never developed far enough to offer a viable alternative to the conventional system.
Prior to the invention of the front derailleur, various solutions using gears had been proposed for changing the ratio in the area of the crankset. These solutions were quickly abandoned and did not achieve the success of the derailleur, in particular due to their cost, the need to have a specific frame, brittleness, and especially poor efficiency. There are new ideas from time to time which use the principle of gears to change the gear ratio in the area of the crankset. The device disclosed in the patent document U.S. Pat. No. 4,218,931 is a step-down crankset. The crankset therein drives a sprocket having external teeth, which meshes with a crown having internal teeth, which is offset with respect to the crankset. The crown is affixed to the toothed chainwheel which receives the transmission chain. The transmission ratio between the pedaling speed and the rotational speed of the chainwheel is determined by the ratio of the diameters of the sprocket and of the crown. In this device, the transmission ratio is fixed and cannot be modified in the area of the crankset.
The patent documents EP 1 980 483 and US 2008/0252037 disclose a crankset having a mechanism for changing the transmission ratio. The mechanism is a planetary gear train which makes it possible to obtain two transmission ratios. The first of these transmission ratios is equal to 1 when the rotation of the planetary gear (the sprocket with external teeth) is free, whereas the second ratio is substantially greater than 1 when the rotation of the planetary gear is blocked. Such a crankset with transmission ratio change has a number of disadvantages. First, taking into account the kinematic constraints of the planetary gear trains, the geometrical constraints of the location of the pedaling axis on a bicycle, it is not possible to make a crankset having a transmission ratio close to 1, i.e., less than 1.15, preferably less than 1.10, in this manner. Indeed, to make such a crankset with a planetary gear train having a ratio close to 1, the satellites must have a number of teeth on the same order of magnitude as the planetary gear, which would result in having a large diameter crankset. However, it is important for the ground clearance to be as substantial as possible.
Furthermore, in view of the large number of mechanical parts, a planetary gear crankset is expensive, heavy, and performs poorly. Moreover, the currently commercially available products which use planetary gear trains are recommended for mountain bicycles dedicated to the downhill disciplines, in which the weight is not such a determinant factor.
Lastly, because it is not possible to obtain a transmission ratio close to 1, the planetary gear cranksets are not easy to use if not coupled to a transmission ratio changing device positioned on the rear wheel. Indeed, for the user, it is desirable for the variations between the gear ratios to be constant and not too far from one another. A range of variations from 5% to 10% between two consecutive gear ratios on road bicycle and between 7% and 15% in VTT is provided by way of example.